A thorough investigation of the emission and absorption spectra of the ( 3 F 4 , 3 H 6 ) band in thulium doped silica fibers has been performed. All the basic parameters of thulium in silica have been extracted with the purpose of further analysis in laser and amplifier simulations. The experimental methods used to obtain the scaled cross sections have been carefully selected in order to avoid problems associated with calibrated measurements and knowledge of the radiative lifetime. The values of the absorption cross sections agree well with previously reported values, however the peak emission to peak absorption cross section ratios are found to be significantly below 1. Also confinement factors and thulium concentrations are estimated from the results. 948-956 (1999). 12. X. Zou and H. Toratani, "Spectroscopic properties and energy transfer in Tm 3+ singly-and Tm 3+ /Ho 3+ doublydoped glasses," J. Non-crystalline Solids 195, 113-124 (1996). 13. B. M. Walsh and N. P. Barnes, "Comparison of Tm:ZBLAN and Tm:silica fiber lasers;Spectroscopy and tunable pulsed laser operation around 1.9 μm," Appl. Phys. B 78, 325-333 (2004).

We have successfully demonstrated a single-frequency distributed-feedback (DFB) thulium-doped silica fiber laser emitting at a wavelength of 1735 nm. The laser cavity is less than 5 cm long and is formed by intracore UV-written Bragg gratings with a phase shift. The laser is pumped at 790 nm from a Ti:sapphire laser and has a threshold pump power of 59 mW. The laser has a maximum output power of 1 mW in a single-frequency, single-polarization radiation mode and is tunable over a few nanometers. To the best of the authors' knowledge, this is the first report of a single-frequency DFB fiber laser that uses thulium as the amplifying medium. The lasing wavelength is the longest demonstrated with DFB fiber lasers and yet is among the shortest obtained for thulium-doped silica fiber lasers.

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